Resolve to discourage people crossing motorway (13)
I believe the answer is:
determination
'resolve' is the definition.
(determination is a kind of resolve)
'discourage people crossing motorway' is the wordplay.
'discourage' becomes 'deter' ('deter' can be a synonym of 'discourage').
'people' becomes 'nation'.
'crossing' indicates putting letters inside.
'motorway' becomes 'mi' (resembles M1, English motorway).
'deter'+'nation'='deternation'
'deternation' placed around 'mi' is 'DETERMINATION'.
'to' acts as a link.
(Other definitions for determination that I've seen before include "legal decision" , "Decision process" , "Firmness of purpose" , "Firm resolve" , "24 [DRIVE]" .)